Motor vehicles are increasingly being equipped with driver assistance systems, e.g., for automatically controlling proximity (ACC, adaptive cruise control) or for warning of imminent collisions and, optionally, for initiating measures for averting the collision or for mitigating the effects of the collision (PSS; predictive safety system). In these driver assistance systems, a radar sensor is used for monitoring the surrounding traffic and, in particular, for locating other vehicles. Since the driver generally relies on the operability of the assistance system, it is necessary, for reasons of safety, that functional impairment of the radar sensor during continuous operation be detected and be able to be signaled to the driver.
For this reason, radar sensors for motor vehicles are equipped with a blindness detection device, which evaluates the received radar signals in a special manner while the radar sensor is operated in the measuring mode, in order to thus generate one or more blindness indicators that indicate functional impairment of the radar sensor, all the way up to complete blinding.
An obvious criterion for blinding consists in that the radar sensor does not receive any radar echoes at all. However, this criterion alone is not very meaningful, since it cannot rule out that no radar targets are actually in the locating range of the sensor.
In general, though, a radar sensor installed in a vehicle always receives certain reflections of ground irregularities. These reflections are referred to as “ground clutter.” The absence of this ground clutter is a relatively reliable indication of blinding.
Heavy rain that reflects and attenuates the radar beam and thereby reduces the sensitivity of the sensor may also be a possible cause of blinding of the radar sensor. For its part, though, the reflection of the radar beam results in a detectable signal, the so-called rain clutter. Thus, in principle, it is possible to detect heavy rain and the functional impairment of the radar sensor accompanying it by purposefully searching for this rain clutter.
Another possible cause of blinding is a layer of dirt or a film of water on the surface of the radar lens or of the radome. Such a layer results in reflection and attenuation of the radar signal and, therefore, in a loss of sensitivity. However, in this instance as well, in principle, the reflection caused by the layer of dirt constitutes, for its part, a detectable signal, with the aid of which the blinding could be detected. Generally, the distance between the layer of dirt and the radar antenna is so short, that it is normally outside of the distance range that may be monitored with the aid of the radar sensor.
Finally, a special form of blinding is the so-called angular blindness, which may be caused by a layer of ice on the radar lens or the radome. To be sure, such an ice layer does not result in significant reflection and attenuation of the radar beam but, due to refraction effects, causes a change in direction of the radar beam, which, in the case of radar sensors having angular resolution, may result in the angle data of the located objects no longer being reliable. The term “blindness detection” is to be understood in a comprehensive sense and shall also include such cases of angular blindness.
The object of the present invention is to provide a radar sensor, in particular for motor vehicles, which renders possible more reliable blindness detection.